Positron computed tomography (PCT) offers two unique advances in the regional assessment of organ function: the isolation of transverse-section imaging and quantitative measurement of the tracer activity. The former intrigues the clinician in need of sharper delineation of the lesion. The latter offers the more far-reaching reward in bridging the gap between nuclear imaging and the more basic sciences underlying the tracer distribution. The missing information needed to convert the 18F-2FDG image into a map of regional glucose metabolism is the time-course of the arterial concentration during the uptake phase. The need for this arterial input function is a common feature generally required for modeling the dynamic response of a tissue to any radiotracer. This project proposes to develop a device capable of non-invasively measuring the arterial concentration of positron-emitters, and to place the instrument into clinical operation assisting current PCT protocols in need of arterial sampling. The arterial monitor isolates the left atrium and ventrical, being the largest arterial pool, by time-of-flight (TOF) techniques on the 511 keV annihilation photon pair. A preclinical prototype has achieved 300 picosecond time resolution (FWHM = 5 cm.), well within the axial dimensions of the great chambers of the left heart. The work proposed centers on upgrading the sensitivity with CsF detectors and engineering the device to meet the rigors of clinical service mounted on the gantry of the ECAT scanner at the combined VA/UW facility. The final phase will follow the clinical application of the TOF-arterial monitor to evaluate its efficacy as it replaces invasive arterial blood sampling in PCT protocols. Any nuclear imaging procedure will keep its diagnostic promise only to the extent that its value far outweighs its insult. Relieving the need for arterial puncture will broaden the application of quantitative PCT procedures in the study of disease and normal function.